1. Field of the Invention
This invention relates to sampling the exhaust gas from the internal combustion engine.
2. Prior Art
As hydrocarbon exhaust gas levels are reduced to meet ever more stringent government regulations, the levels in vehicle exhaust gas actually approach those in ambient air. This makes accurate measurements of exhaust species at these lower concentration levels more difficult and requires the development of new measurement and sampling technologies. One solution to the sampling issue is use of what is referred to as a "mini-diluter." The analyses themselves are carried out on diluted exhaust gas (to prevent water condensation) at close to ambient temperature. To avoid condensation it is necessary to keep the exhaust gas temperature above its dew point until the dilution is made.
The current solution to this problem is to use heated sample lines. During "cold start," the connection between the cold exhaust pipe and the heated sample line is cold and may take a few minutes or more to heat up because of the low sample flow rate. Even if the connection between the heated sample line and the exhaust pipe is heated with a heating tape, it is hard to heat the very end of the connection. The probe described in this patent application avoids this problem.
Further, it is necessary to maintain the sampled exhaust gas in a heated state until both the mass of the sample is known and the sample has been diluted so that it does not have to be maintained in a heated state to be accurately analyzed. Further, it is known to measure the exhaust flow using two hot wire anemometers, one in the make up air, and the other in the diluted exhaust. This method of measuring the exhaust flow has the inherent error of subtracting two large values which are almost equal at idle and low flow conditions.
Referring to FIG. 1, in the prior art, a heated sample line 10 is about 7 to 10 feet long and is coupled to an oven 11 which contains a mass flow meter 12. After mass flow meter 12 measures the sample exhaust gas flow in the heated sample line 10, a diluting gas from a nitrogen mass flow meter 13 is added and flow proceeds down a tube 14 to a pump 15 and into a sample bag 16. It would be desirable to be able to substantially eliminate heated sample line 10, oven 11, and mass flow meter 12. These are some of the advantages the invention provides.
In summary, a mini-diluter is used to obtain a representative sample of engine exhaust gas for the purpose of calculating the mass of various emissions during an engine or vehicle test. This is achieved by taking an exhaust gas sample whose flow rate is proportional to the exhaust flow rate of the engine.
To avoid condensation of the water vapor in the exhaust the exhaust gas has to be diluted with dry gas such as nitrogen or zero air so that the dew point of the diluted mixture is lower than room temperature. To keep the concentration of the components to be measured high the dilution should not be greater than necessary. This means the dilution gas flow should also be proportional to the exhaust flow.
The advantages of a mini-diluter over other prior art systems include:
1. The dilution is made with pure gases rather than room air so background corrections are not needed. PA1 2. The sample can be drawn anywhere in the exhaust system (such as feedgas or midbed) not just at the tailpipe. PA1 3. A smaller amount of exhaust sample has to be handled (e.g. 0.1% versus 100%); no cooling or water trap are needed. PA1 4. The concentration of exhaust gas in the mini-diluter bag can be 3 or 4 times that in other prior art system bags since the dilution is constant during the test. This makes the job of analysis easier. PA1 1. Heated capillary can be inserted into the exhaust pipe leaving no cool connection for exhaust gas sample condensation. PA1 2. The heated capillary does not need an oven like the sample mass flow meter does. PA1 3. The diluting gas can be added at a "T" at the end of the heated capillary. The heated sample line between the sample point and mini-diluter is not necessary. PA1 4. The capillary pressure drop signal used for measuring the sample flow is about 20 times faster than prior art mass flowmeter signals. This allows for fast control of the sample and diluent flows in the mini-diluter system. PA1 5. The sampling capillary can easily be kept at 200.degree. or 300.degree. C. Also the temperature can be raised to burn off any deposits. PA1 6. The power requirements are about 10 times less than the oven and heated line. PA1 7. The capillary is at operating temperature in a few seconds. PA1 8. The inner diameter of the capillary is about 1 mm compared to about 0.2 mm for the mass flowmeter sensor tube. Therefore a filter will not be needed to protect the heated capillary. PA1 9. Since the heated capillary is right at the sampling point there is no need to delay the start and stop of sampling to assure that the proper sample gets into the sampling bags. This is because the diluted sample travels much faster and its delay can be neglected.